More than 50 years after a partial meltdown of a nuclear reactor in the hills between Chatsworth and Simi Valley, new federal data show that radioactive material remains in the soil despite previous attempts at cleanups.

The data, released to the public Wednesday by the federal Environmental Protection Administration, show that some samples of surface soil collected at the Santa Susana Field Laboratory site exceed standards outlined in a 2010 agreement between state and federal agencies.

Cleanup activists and property owner the Boeing Co. disagreed over the significance of the findings.

Denise Duffield, associate director for Physicians for Social Responsibility-Los Angeles, which works alongside the Rocketdyne Cleanup Coalition, said it shows that previous remediation efforts have failed.

"These astonishing findings reinforce the absolute necessity of the promised cleanup going forward fully and without delay," Duffield said. "Together and with other like-minded organizations and community members, we will continue to resist Boeing and its allies, in and out of agencies, who have tried to undermine the binding cleanup agreements.

"Half a century after the meltdown, the neighboring communities have a right for all this contamination to finally be cleaned up," she added.

But Boeing officials said Wednesday they were pleased the survey was completed and said it shows the radioactive contamination is minimal.

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"The survey results show that the radiation found is low-level, poses no significant health risk and occurs in a small number of former operational areas," Boeing said in a written statement. "Their findings confirm that prior remediation efforts were effective at eliminating the majority of the radiological contamination from the former federal government energy research facility.

Acting as independent monitors, scientists with the EPA strapped gamma scanners on bulldozers, three-wheeled contraptions and even mules to conduct radiological surveys on a portion of the land known as Area IV, where the nation's first nuclear reactor was built to produce electrical power for a commercial grid. In 1959, the reactor experienced a partial meltdown.

The EPA researchers collected 3,735 samples of mostly surface soil and found that of those, 500 contained concentrations of radioactive materials that exceeded what is known as background standards - or the levels occurring naturally in the environment. Almost all were man-made radionuclides. Most of those samples contained Cesium-137, and of those one sample reached levels up to 1,000 times above background standard. There were 153 samples of Stronium-90 and of those some hits reached levels that were 284 times higher than background.

Both radioactive elements are considered dangerous to human health when present at high levels.

"There were some hits that were elevated but for the most part, they were in the range that we expected," said John Jones, federal project director with the Department of Energy.

In 2010, both the DOE and NASA agreed to adhere to strict state standards to decontaminate their portion of the 2,849-acre property nestled between Chatsworth and Simi Valley. The majority of the land is owned by Boeing, which acquired it as part of its purchase of Rocketdyne. (Boeing later sold Rocketdyne but kept the Santa Susana Field Lab property.)

Longtime residents and activists with the Rocketdyne Clean-Up Coalition and the Committee to Bridge the Gap, as well as public officials who have fought for years for the removal of radiation and chemical contaminants at the site, said the data offer much needed evidence that cleanup to standards found in state law SB 990 - the highest in the nation - must be completed for the whole property.

Boeing, however, has argued against decontaminating its property to the strictest state standards, saying it would destroy the natural resources on the site. A federal judge sided with Boeing last year, allowing the company to pursue a cleanup to more moderate federal standards.

A full assessment of the entire Santa Susana Field Lab, including groundwater, has yet to be completed.

After preliminary data from this latest study were released in March, activists and politicians called the findings disappointing, "especially because the site has already undergone two cleanup efforts by its owner, The Boeing Co. and the Department of Energy. Each declared the land fully cleaned," Assemblywoman Julia Brownley said at that time.

Jones said the cleanups that occurred then were completed to standards that were set at the time.

"What the activists say is true," Jones said. "They (owners) cleaned up to a previously agreed level. We want to clean up to a stricter level."

Both the state Department of Toxic Substances Control and the DOE will now work to complete a chemical characterization study. Afterward, a decision document will be developed to address cleanup. The cleanup is expected to be completed by 2017 under the 2010 agreement.

"These results simply give us a better idea of where the radionuclide areas of interest are," said Mark Malinowski, project team manager with the DTSC. "This is a complex site and there is still work to do before cleanup begins."

The EPA received $41.5 million of DOE and Recovery Act Funds to conduct what they called in their report "one of the most comprehensive technical investigations ever undertaken for low-level radioactive contamination."